1. Field of the Invention
The present invention relates to a shift register of a display device and a liquid crystal display (LCD) using the same, and more particularly, to a shift register generating a scanning signal for scanning gate lines in a gate line driving circuit of AMTFT-LCD (Active Matrix Thin Film Transistor Liquid Crystal Display) and for selecting a block of data lines in a data line block driving circuit.
2. Description of the Related Art
In these days, information processing devices have been rapidly developed in a trend with various architectures, functions and faster information processing speed. Information processed in these information processing devices has an electrical signal format. In order to visually confirm information processed in the information processing device, a display for a role as an interface should be provided.
LCDs have advantages such as the lighter weight, small size, high resolution, lower power consumption and friendly relation with environment and they render display of full colors compared with the traditional cathode ray tube (CRT). Such the advantages allow the LCDs to replace the CRTs and to be spotlighted as a next generation display.
LCDs apply an electric power to liquid crystal having a specific molecular configuration to vary the molecular arrangement of liquid crystal. The variation in the molecular configuration of liquid crystal causes a variation in optical properties such as birefringence, optical rotary power, dichroism and light scattering. The LCDs utilize such variations in optical properties to display an image.
The LCD deyice is largely sorted into a TN (Twisted Nematic) type and a STN (Super-Twisted Nematic) type. The liquid crystal display device is, according to the driving method, sorted into an active matrix display type, which uses a switching device and a TN liquid crystal, and a passive matrix type, which uses an STN liquid s crystal.
A distinguishable difference of two types is in that the active matrix display type is applied to a TFT-LCD that drives the LCD by using a TFT and the passive matrix display type dispenses with a complicated circuit associated with a transistor because of using no transistor
TFT-LCD is divided into amorphous silicon TFT LCD (a-Si TFT-LCD) and polycrystalline silicon TFT LCD (poly-Si TFT-LCD). Poly-Si TFT-LCD has advantages of lower power consumption, lower price compared with a-Si TFT-LCD but has a drawback in that its manufacturing process is complicated. Thus, poly-Si TFT-LCD is mainly used in a small sized display such as mobile phones.
Amorphous-Si TFT-LCD is applied to a large screen sized display such as notebook personal computer (PC), LCD monitor, high definition (HD) television, etc., due to easy application of large screen and high production yield.
As shown in FIG. 1, poly-Si TFT LCD includes a data driving circuit 12 and a gate driving circuit 14 that are formed on a glass substrate 10 having a pixel array. A terminal part 16 is connected with an integrated printed circuit board (PCB) 20 using a film cable 18. This structure saves the manufacturing costs and minimizes the power loss due to the integration of driving circuits.
As shown in FIG. 2, amorphous-Si TFT LCD has a data driving chip 34 formed on a flexible printed circuit board 32 in a manner of COF (Chip-On-Film). A data printed circuit board 36 is connected with a data line terminal of the pixel array through the flexible PCB 32. A gate driving chip 40 is formed on a flexible PCB 38 in a manner of COF. A gate PCB 42 is connected with a gate line terminal through the flexible PCB 40.
Recently, there is also disclosed a technique for removing the gate PCB from the LCD by employing an integrated PCB technique mounting a gate power supply part on the data PCB. Korean Patent Laid-Open Publication No. 2000-66493 previously filed by the present assignee discloses an LCD module employing an integrated PCB by which the gate PCB is removed.
However, although the integrated PCB is employed, the flexible PCB having the flexible PCB is still used. Thus, since multiple flexible PCBs should be assembled in a glass substrate, a fabrication process of a-Si TFT LCD, especially OLB (Outer Lead Bonding) process becomes complicated compared with that of poly-Si TFT LCD, whereby its fabrication costs becomes higher.
Thus, there are many endeavors for decreasing the number of the assembly process by simultaneously forming data driving circuit and gate driving circuit along with pixel array on a glass substrate in a-Si TFT LCD like poly-Si TFT LCD.
U.S. Pat. No. 5,517,542 discloses a technique for an a-Si TFT gate driving circuit formed on a glass substrate.
In the above U.S. Pat. No. 5,517,542, a shift register of the gate driving circuit uses three clock signals. Each stage of the shift register two clock signals of three clock signals, is enabled with an output signal of previous stage as an input signal, and maintains the disable state by feed backing an output of the second next stage.
Each stage in the US Patent provides a voltage applied to gate of full-down transistor in a capacitor charge manner in order to maintain the disable status. Thus, when an increase in gate threshold voltage of the full-down transistor due to stress of the full-down transistor is elevated higher than the charge voltage of the capacitor, there may occur a turn-off of the full-down transistor in the disable status.
The above US patent employs a power supply circuit elevating a VDD in proportional to the increase in the threshold voltage of the a-Si TFT LCD in order to prevent an error operation due to the increase in the threshold voltage.